The three-dimensional structures of various nucleotide binding proteins, in particular, of NAD ion linked dehydrogenases are being investigated. Those domains, within each enzyme, involved in binding coenzyme are mostly of similar tertiary structure, although of rather different primary structure. Such structures have been studied in lactate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase. The catalytic mechanism for both these enzymes is being studied both by investigating certain structures carefully (dog-fish M4 LDH and lobster GPD) and also by comparing these with other isozymes in other species.